Can I use buttermilk in this recipe, instead of heavy cream?

Rather late to comment but ...definitely YES !
Lily 🌹.

Buttermilk may make the filling too runny. I think the flavor would be awesome, though. I would suggest separately beating very cold buttermilk in a chilled bowl with chilled beaters. Then fold into the cream cheese mixture. Pretty much following Eric Kim's directions for making this by hand.

Let us know if you try this :)